What do fungi and arthropods have in common?

A) Both groups are commonly coenocytic.

B) The haploid state is dominant in both groups.

C) Both groups are predominantly heterotrophs that ingest their food.

D) The protective coats of both groups are made of chitin.

E) Both groups have cell walls.

Diagram of the chemical structure of chitin

The correct answer is D) The protective coats of both groups are made of chitin.

Both arthropods and fungi are organisms which rely on the presence of chitin to provide strength and protection. The chitin in both cases forms bonds with other molecules to provide a rough material that helps adapt these organisms to their environment.

Arthropods are considered to be the most successful of all animals on earth. Much of this success is due to the exoskeleton which consists of epicuticle and procuticle. The procuticle is the part of the integument that contains chitin that bonds with various proteins.

The exoskeleton of these animals has enabled them to have jointed appendages which is important for movement. Some of these arthropods, such as crustaceans even impregnate the chitin layer with calcium carbonate to provide increased strength and hardness.

The disadvantage of the chitinous exoskeleton is that it does not grow with the animal, thus this means that arthropods have to molt and deposit a new layer of cuticle. This is when the animal is most vulnerable to predators and environmental stresses.

Fungi have cell walls that contain chitin which bonds in complex ways with various other molecules including glucans, mucopolysaccharides and sometimes other substances. This provides a rough outer layer for the cells that also gives the fungal body strength, support, and porosity.

Eukaryotic organisms

Arthropods and fungi are both organisms that are made of eukaryotic cells. While the arthropods are found within the Animal kingdom, the fungi are their own kingdom of life.

These organisms have tissues that are made of cells which contain membrane-bound organelles. While other eukaryotic organisms such as plants contain cellulose in the cell wall, fungi contain chitin instead. This chitin interacts with other molecules in the wall to provide strength and support.

Animal cells do not have a cell wall, but in the case of invertebrates, there are cells that produce chitin that forms a protective layer and is part of the exoskeleton of the animal. The chitin links with other molecules to provide a strong structure.

Arthropoda

This group of animals can be characterized by the presence of jointed appendages including limbs and segmented sections of the body. It is a very large and diverse group of animals which includes millipedes, centipedes, all the crustaceans and all of the insects.

The arthropoda are, in fact, the most successful of all living animals in terms of numbers and diversity, and they are found in every habitat on earth.

The number of body segments and the number of appendages do vary among the different animals found within the arthropoda. The main reason these organisms are able to have jointed legs and body tagmata is due to the features of the exoskeleton.

Exoskeleton

The exoskeleton of arthropods is secreted by the cells of the epidermis in the integument. This exoskeleton is made of protein and chitin covered by a layer of wax that functions to waterproof the structure.

The waxy layer on the outermost part of the cuticle prevents the loss of water from the integument, which is important in species of arthropods that live on land. Insects that are terrestrial have spiracles, which are holes that open into the tracheal system.

The protein layer is known as the epicuticle and the chitin layer is located beneath this and is known as the procuticle. In some arthropods, such as crustaceans, calcium carbonate ions are also deposited into the procuticle to provide added strength and hardness.

Chitin is a large molecule that consists of N-acetyl-d-glucosamine residues that are linked together by β-(1,4)-glycosidic linkage.

This is a tough substance that provides support, protection, and area for the attachment of muscles and tissues. It often cross-links with protein fibers to form a tough, impenetrable layer.

A major drawback of having an exoskeleton of chitin is that it does not grow when the rest of the body grows. This means that the organism has to shed the exoskeleton on occasion and then deposit a new layer of tissue.

It is during this molting period that arthropods are very vulnerable to environmental conditions and predators.

The chitin is not deposited evenly in the exoskeleton and at the joints, the layers are thin and more flexible to enable movement of the body to occur.

Fungi

The fungi are one of the kingdoms of life that contain about 144,000 species of all sizes and shapes. The group includes mildews, mold, yeast, rusts and mushrooms.

The fungal cell wall is comprised of a number of substances, one of which is chitin. The acetylglucosamine polymers making up the chitin associate with other substances in the wall, including mucopolysaccharides, waxes, glucans and various pigment molecules.

These molecules work together to ensure that the organism has tough, and fairly rigid cell walls that provide strength and support.

The chitin often forms cross-links with branches of the β-1,3 glucan. It also bonds to other complex carbohydrate molecules including β-1,6 glucan and galactomannan.

It is believed that the glucans and chitin found in the cell wall of the yeast Candida albicans are responsible for the shape and strength of the wall, while the mannoproteins provide some degree of porosity.

There is variability with different species of fungus having slight differences in the exact makeup of the cell wall. The growth stage of the fungal species is also important in determining the composition of the wall.

Scientists are keen to continue researching the structure of the fungal cell wall because this can be used as a target for antifungal medications.

Humans do not have cell walls around their cells, which means that a treatment that is designed to attack the cell wall of pathogenic fungi should be relatively safe for people.

References

  1. RD Barnes (2019). Arthropod. Retrieved from Encyclopedia Britannica.
  2. S Das et al. (2003). Chitin structure and complexes. Retrieved from sciencedirect.com.
  3. Editors of Encyclopedia Britannica (2019). Exoskeleton. Retrieved from Encyclopedia Britannica.
  4. RL Dorit, WF Walker, RD Barnes (1991). Zoology. Philadelphia: USA, Saunders College Publishing.
  5. V Amidhajian, D Moore, C Alexopoulos (2019). Fungus. Retrieved from Encyclopedia Britannica.

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